In recent years, organic electroluminescent elements using organic materials as hole-transport materials or charge-transport materials constituting the electroluminescent elements have been intensively studied.
As such charge-transport materials, for example, anthracene derivatives, anthraquinoline derivatives, imidazole derivatives, styryl derivatives, hydrazone derivatives, triphenylamine compounds, poly(N-vinylcarbazoles), and oxadiazoles are known.
Liquid-crystal compounds have been used as materials for displays and applied to various devices, such as clocks, desktop electronic calculators, television sets, personal computers, and cellular phones. The liquid-crystal materials are classified into thermotropic liquid crystals (liquid crystals in which transitions depend on temperature) and lyotropic liquid crystals (liquid crystals in which transitions depend on concentration) on the basis of the mechanisms of phase transitions. From the standpoint of molecular arrangements, these liquid crystals are classified into three groups: smectic liquid crystals, nematic liquid crystals, and cholesteric liquid crystals. The liquid crystals are also known as anisotropic liquids and exhibit optical anisotropy similarly to optically uniaxial crystals. Observation using an orthoscope is usually performed between crossed Nicols, and is useful for the identification of types of liquid crystals and for the determination of the transition temperatures of liquid-crystal phases. The smectic liquid crystals are classified into A, B, C, D, E, F, and G on the basis of characteristic birefringent optical patterns observed with the orthoscope.
Hanna et al. have found that liquid-crystal compounds having smectic phases are capable of transporting charges and have proposed charge-transport materials using the liquid crystal compounds. They have proposed, for example, a liquid-crystalline charge-transport material exhibiting smectic liquid crystallinity and having a reduction potential of −0.3 to −0.6 (V vs. SEC) with reference to a standard calomel electrode (SEC) (Japanese Unexamined Patent Application Publication No. 09-316442); a liquid-crystalline charge-transport material including a liquid crystalline compound exhibiting a smectic phase having self-orientation properties and a predetermined amount of fullerene C70 having a sensitizing effect (Japanese Unexamined Patent Application Publication No. 11-162648); a polymer membrane in which a liquid-crystalline charge-transport material is dispersed in the polymer matrix, in other words, a polymer membrane in which a liquid-crystalline compound exhibiting a smectic phase is dispersed (Japanese Unexamined Patent Application Publication No. 11-172118); a liquid-crystalline charge-transport material including a mixture containing a smectic liquid-crystalline compound (Japanese Unexamined Patent Application Publication No. 11-199871); a liquid-crystalline charge-transport material having smectic liquid crystallinity and having an electron mobility or hole mobility of 1×10−5 cm2/v·s or more (Japanese Unexamined Patent Application Publication No. 10-312711); and a liquid-crystalline charge-transport material including a smectic liquid crystalline compound having, in one molecule, a functional group capable of forming a new intermolecular or intramolecular bond and a functional group capable of transporting a hole and/or electron (Japanese Unexamined Patent Application Publication No. 11-209761).
Smectic liquid-crystalline compounds disclosed in the above-described Patent Documents include smectic liquid-crystalline compounds each having a 6-π-electron aromatic ring such as a benzene ring, a pyridine ring, a pyrimidine ring, a pyridazine ring, a pyrazine ring, or a tropolone ring; smectic liquid-crystalline compounds each having a 10-π-electron aromatic ring such as a naphthalene ring, azulene ring, a benzofuran ring, an indole ring, an indazole ring, a benzothiazole ring, a benzoxazole ring, a benzimidazole ring, a quinoline ring, an isoquinoline ring, a quinazoline ring, or a quinoxaline ring; and smectic liquid-crystalline compounds each having a 14-π-electron aromatic ring such as a phenanthrene ring, or an anthracene ring. In these compounds, charges are transported in a smectic-A phase. However, the above-described method for transporting charges requires photoexcitation. Furthermore, conductivity is 10−13 s/cm without photoexcitation and 10−11 s/cm in a photoexcited state. The conductivity values are the same levels as those of an insulating material.